This invention relates to a process for removing and recovering sulfur dioxide from sulfur dioxide-containing exhaust gas with good efficiency. More particularly, it relates to a wet process for the desulfurization of exhaust gas which comprises contacting the exhaust gas with an absorbent mainly containing potassium salts of tartaric acid to absorb sulfur dioxide in the exhaust gas and then heating the absorbent to recover sulfur dioxide therefrom efficiently and to regenerate it, and finally circulating the absorbent through the absorbing step.
Gas, which is exhausted from various boilers, heating furnace, combustion furnace and the like, contains sulfur dioxide as an air pollutant. In order to prevent air pollution caused by sulfur dioxide, the desulfurization of exhaust gas has been studied and many desulfurizing devices are in operation.
The desulfurization of exhaust gas can be classified generally as a dry process or as a wet process. Recently, the latter is predominantly employed, because it can attain a high desulfurizing rate easily with a compact device. Most of the currently commercialized processes are wet process.
Wet processes are further classified according to the kind of the recovered by-product such as gypsum, sulfur, sulfur dioxide, sodium sulfite, sodium sulfate, ammonium sulfate and the like.
This invention is a wet process for recovering sulfur dioxide among the processes described above.
Various wet processes for desulfurization by recovering concentrated sulfur dioxide from a dilute sulfur dioxide in the exhaust gas, have been proposed. Wellman-Lord's method and the MgO method have been commercially performed. The former method consists of absorbing sulfur dioxide contained in exhaust gas into a solution of sodium sulfite according to the equation (1), EQU Na.sub.2 SO.sub.3 +SO.sub.2 +H.sub.2 O.fwdarw.2NaHSO.sub.3 ( 1)
stripping SO.sub.2 by heating and concentrating a resulting solution containing NaHSO.sub.3 and Na.sub.2 SO.sub.3 according to the equation (2), EQU 2NaHSO.sub.3 .fwdarw.Na.sub.2 SO.sub.3 +SO.sub.2 +H.sub.2 O (2)
recovering the concentrated SO.sub.2 gas, dissolving Na.sub.2 SO.sub.3 precipitated by concentration in water and then circulating it through the absorption system.
The latter method consists of absorbing sulfur dioxide contained in exhaust gas in a slurry of Mg(OH).sub.2 according to the equation (3), EQU Mg(OH).sub.2 +SO.sub.2 +5H.sub.2 O.fwdarw.MgSO.sub.3.6H.sub.2 O (3)
separating the resulting crystal of MgSO.sub.3.6H.sub.2 O, decomposing it by calcination according to the equation (4), EQU MgSO.sub.3.6H.sub.2 O.fwdarw.MgO+SO.sub.2 +6H.sub.2 O (4)
recovering concentrated SO.sub.2 and circulating MgO to the absorption system to slake it into Mg(OH).sub.2 which is used again for the desulfurization.
However, the absorbent used in the former method is preferable for the absorption of sulfur dioxide, but not always suitable for its liberation, as the effect of the increase in equilibrium partial pressure of sulfur dioxide by elevation of temperature is small as compared with the other alkali sulfites, and even if the increase in equilibrium partial pressure is promoted by concentrating the absorbent and precipitating some Na.sub.2 SO.sub.3, the amount of steam which is consumed is still very large. On the other hand, the latter method needs a large amount of fuel for the decomposition of MgSO.sub.3 and the evaporation of water of crystallization and adhesive moisture. Therefore, it can not be said that the both methods are efficient, because they consume a large amount of energy for the liberation of sulfur dioxide.